Both are temperature sensors placed inside the winding, but the information they provide differs. A PTC thermistor is a threshold element: its resistance is low up to its rated trip temperature and rises rapidly once the threshold is reached. A separate thermistor protection relay detects this jump and de-energises the contactor coil. A PT100 is a platinum resistor whose value changes smoothly with temperature; it shows the winding temperature in degrees.

FeaturePTC thermistorPT100
Information givenThreshold exceeded / not exceededContinuous temperature value
Connected toThermistor relay or a drive with this inputPT100 measuring module, temperature relay or PLC analogue input
Warning and tripSingle threshold; a second sensor set is needed for early warningThresholds freely set on the device
Typical useGeneral industrial protectionMonitoring of large or critical motors

Connection notes: the sensor leads are brought out to separate auxiliary terminals in the terminal box; over long distances they should run separately from the power cable, preferably in shielded cable. Do not apply high voltage to PTC leads with an insulation tester. Fitting the sensor in the winding during manufacture is far more practical than adding it later. PTC/PT100 and the anti-condensation heater are order options; mention them in your quote request. The operational temperature monitoring side of the sensor is covered in the motor maintenance and overheating guide.